Intellectual disability

Intellectual disability (ID), also known as general learning disability in the United Kingdom and formerly mental retardation, is a generalized neurodevelopmental disorder characterized by significantly impaired intellectual and adaptive functioning. It is defined by an IQ under 70, in addition to deficits in two or more adaptive behaviors that affect everyday, general living. Intellectual functions are defined under the DSM-5 as reasoning, problem‑solving, planning, abstract thinking, judgment, academic learning, and learning from instruction and experience, and practical understanding confirmed by both clinical assessment and standardized tests. Adaptive behavior is defined in terms of conceptual, social, and practical skills involving tasks performed by people in their everyday lives.

Intellectual disability is subdivided into syndromic intellectual disability, in which intellectual deficits associated with other medical and behavioral signs and symptoms are present, and non-syndromic intellectual disability, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.

Intellectual disability affects about 2 to 3% of the general population. Seventy-five to ninety percent of the affected people have mild intellectual disability. Non-syndromic, or idiopathic cases account for 30 to 50% of these cases. About a quarter of cases are caused by a genetic disorder, and about 5% of cases are inherited. Cases of unknown cause affect about 95 million people as of 2013.